US scientists say crops can pass on to related weeds genetic traits that persist for six generations or more.

It's inevitable that these and other fitness-related traits will make their way into weed populations

Prof Allison Snow

This may make the weeds harder to kill, and more resistant to pests like insects.

The scientists say it is inevitable that characteristics designed to improve crops will make their way into weeds.

They say biotechnology companies should avoid developing crop varieties capable of helping weeds in this way.

The scientists, based at Ohio State University studied the behaviour of cultivated and wild radishes. The results were presented to the annual meeting of the Ecological Society of America by Allison Snow, professor of ecology at Ohio.

Transfer inevitable

Professor Snow said her team had found that genetic traits developed in crops - resistance to insect pests, for example - could become a permanent part of the weed population.

The new hybrid weeds might not at first be as fit as their wild parents, but they seemed to regain reproductive fitness rapidly.

Each crop "needs judging differently"

Professor Snow said: "It's inevitable that these and other fitness-related traits will make their way into weed populations. The result may be very hardy, hard-to-kill weeds.

"Gene movement from crops to their wild relatives is an ongoing process that can spur rapid evolutionary adaptation in weeds that will ultimately be harmful to crops."

The researchers studied four populations of hybrid and wild radishes in Michigan over six years.

Movement of traits

At the start, each field contained 100 first-generation crop-wild radish hybrids, and 100 wild radishes.

This confirms all the worries we've had about genetically modified (GM) plant technology

Michele Burton, UK Soil Association

In order to monitor the continuation of crop radish genes, the scientists looked for four genetic traits - two enzymes, flower colour, and pollen fertility.

On average, the wild plants reached their peak flowering a month earlier than the hybrids, which also produced fewer seeds per fruit and fewer viable pollen grains than the wild radishes.

Many hybrids (60-78%) produced no fruit, while 92-97% of the wild plants did. But characteristics from the original crop, like white flower colour, persisted in subsequent generations of hybrids.

Professor Snow said: "Even though the effects of delayed flowering and reduced fertility inhibited the movement of certain crop traits to later generations, we did find evidence of crop genes in every generation."

Fertility compared

The team concluded that genetic traits can persist for at least six generations, and probably for much longer.

To compare the lifetime fertility of wild and hybrid plants, they also grew one population of potted radishes.

GM crops continue to arouse distrust

In these plants, viable pollen averaged 63%, compared with 92% in wild plants.

Although the potted hybrids flowered on average half a month later than the potted wild ones, they still produced enough viable pollen with enough time left in the growing season for pollination to occur.

Professor Snow said: "The hybrids were capable of ecologically significant levels of reproduction.

"The constant gene flow between crops and weeds is a subtle process that no-one may notice, but evolution can happen very quickly."